Focus control for television image tubes



Jan. 6, 1948.

M. CAWElN FOCUS CONTROL FOR TELEVISION IMAGE TUBES Filed Nov. 19, 1943 HORIZONTAL vn'ncm. l8 DEFLECTION DEFLECTION VOLTAGE VOLTAGE SIGNAL SOURCE 3 4 5 I 6 7 ATTORNEY INVENTOR MADISON CAWEIN Patented Jan. 6, 1948 FOCUS CONTROL FOR TELEVISION IMAGE TUBES Madison Cawein, Fort Wayne, Ind., assignor, by mesne assignments, to Farnsworth Research Corporation, a corporation of Indiana Application November 19, 1943, Serial No. 510,969

6 Claims.

This invention relates to cathode ray tubes and particularly to the control of electrode potentials for such tubes.

A conventional arrangement for operating cathode ray tubes, whereby the various electrode potentials are adjusted to suit the particular use, is one in which the electrode potentials are derived from a voltage divider which is energized from a common source of electrical energy. Where adjustment of one or more of the electrode potentials is required, such a feature is provided by a movable contact on the appropriate voltage divider'element.

As is well known in the art, however, such arrangements are not particularly well suited for installations wherein a remote or automatic control of the electrode potentials is required. Also, since some of these potentials are of the order of several thousand volts, it ordinarily is impractical to utilize a potentiometer for effecting a voltage variation of an electrode. A device of this character necessarily must be of considerable size and consequentlywill have considerable weight.

There are many uses for cathode ray tubes in which an electrode potential adjustment is necessary where space and weight requirements will not permit of the use of a potentiometer capable of withstanding such high voltages.

It is an object of the present invention, therefore, to provide a novel electrode potential control for a cathode ray tube, whereby to facilitate adjustment of one of the electrode potentials.

In accordance with the invention, there is provided, for a cathode ray tube having a source of electrons and a focusing anode, a relatively high resistance element through which current is conducted to develop a potential for impression upon the focusing anode. The magnitude of the current flow through the high resistance element is controlled by electronic means, the conductivity of which may be varied as desired.

In a preferred embodiment of the invention disclosed herein, the high resistance element is in the form of a wall coating for the tube. Such a structure not only reduces the number and weight of the circuit components necessary externally of the tube, but also serves to improve the operation of the cathode ray tube under certain conditions. In order to vary the magnitude of the current flow through the high resistance wall coating there is provided externally of the cathode ray tube a current controlling device such as a vacuum tube.

For a better understanding of the invention, together with other and further objects thereof,

reference is had to the following description, taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

The single figure of the drawing illustrates diagrammatically a cathode ray tube and a circuit arrangement embodying the invention.

Having reference now to the drawing, the cathode ray tube is provided with an envelope I upon the inside surface of one end wall of which is formed a fluorescent screen 2. Adjacent the opposite end of the tube envelope there is provided a source of electrons such as a cathode 3 which may be heated conventionally by means (not shown) to provide a copious quantity of electrons. The tube also is provided with an electron control element such as a grid 4. A focusing anode 5 is embodied in the tube envelope for the purpose of forming an electron beam. The electron beam is defiected over the surface of the fluorescent screen 2 by conventional means such as horizontal and vertical deflecting plates 6 and 1, respectively.

From a point adjacent the anode 5 to a point adjacent the fluorescent screen 2 the inner wall of the tube envelope 1 is provided with a coating 8. This wall coating is of a character having a relatively high resistance of the order of 200 megohms. It is contemplated that it may be formed on the wall in any one of a number of known ways such as that disclosed in Patent No. 2,183,302, granted on December 12, 1939, to Gerhard Brauer, and entitled Method for producing coatings of high ohmic resistance in the interior of Vacuum tubes. The end of the wall coating adjacent to the anode 5 is electrically connected to the anode by a conductor 9 or in any other suitable manner. The end of the wall coating adjacent to the fluorescent screen 2 is connected to the positive terminal of a source of unidirectional electrical energy such as a battery ID of which the negative terminal is connected to ground.

The focusing anode 5 is connected to the anode of a vacuum tube ll having its cathode connected through a potentiometer I2 to ground. The space discharge path of the tube is shunted for alternating current by a by-pass condenser l3 connected between the tube anode and ground. The control grid I4 of the tube also is connected to ground and the suppressor grid [5 is connected to the cathode. The screen grid l6 of the tube is connected through a resistor I! to an intermediate point of the battery [0. The vacuum tube I I may be a 6AL6G type or any equivalent capable 3 of withstanding anode-to-cathode voltages of the order of 10,000 volts. The focusing anode 5 of the cathode ray tube is maintained at a suitable positive potential with respect to the cathode 3 by the voltage developed across the tube H and the potentiometer l2.

The signals which it is desiredto reproduce upon the fluorescent screen of the cathode ray tube and which are derived from a signal source I 8 are impressed upon the cathode 3 and the control grid 4. Suitable horizontal "and vertical d'e: flection voltages derived from sources l9 and '20 are impressed, respectively, upon horizontal and vertical deflection plates 6 and 1, whereby to-deflect the electron beam over the surface of the fluorescent screen according to any desired pattern.

Referring now to the operation of the invention, it is assumed that the flow of electrons from the cathode 3 to the fluorescent screen 2 is modulated by the control grid 4 under the control of signals derived from the source H3. The flow of electrons is focused into a beam having the desired cross sectional configuration by the focusing anode 5. The functioning of this anode is controlled in part by the potential which is impressed upon it. It is seen that there is provided a series circuit including the battery l0, the high resistance wall coating 8, the space discharge path of the vacuum tube l l and the potentiometer l2. The current flow in this circuit is determined principally by the voltage of the battery ID, the resistance of the Wall coating 8 and the impedance of the space discharge path of the vacuum tube H.

The potential impressed upon the focusing anode 5 is numerically equal to the voltage of the battery [0 minus the voltage drop in the wall coating 8. This voltage drop in turn is determined by the magnitude of the current flowing in the wall coating. The magnitude of this current is determined in part by the impedance of the vacuum tube II. The potential of the control grid 14 of this tube, With respect to its associated cathode determines the impedance of the vacuum tube. This potential may be varied in any one of a number of well known ways, one of which is indicated herein as the potentiometer I2.

When it is desired to readjust the potential impressed upon the focusing anode 5, a suitable adjustment of the potentiometer I2 is made to change the magnitude of the current flowing in the series circuit. Obviously, such a change produces a corresponding change in the voltage drop in the wall coating 8 whereby to eiTect the desired adjustment of the focusing anode potential.

It frequently is convenient to utilize a high resistance wall coating for the cathode ray tube of a high vacuum type so that throughout the length of the wall coating there is established a potential gradient. This potential gradient may be made uniform throughout the length of the Wall coating or may be established according to any other predetermined pattern'as desired. An advantage of utilizing such a wall coating is to facilitate theovercoming of wall charges in high vacuum cathode ray tubes.

Where such a structure is to be used in a tube -requiring adjustment of the focusing anode potential, apparatus such as that embodying the instant invention is particularly useful for effecting the anode potential adjustment.

However, it is not contemplated-that the novel features and beneficial results to be obtained by the use of apparatus embodying this invention are 4 restricted to the use of the disclosed type of high resistance element. An external resistor may be substituted for the high resistance Wall coating in an obvious manner without departing from the scope of the invention.

While there has been described what, at present, is considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In combination, a cathode ray tube having a source of electrons and an electron focusing anode, a; source of electrical energy, means including a relatively high resistance wall coating for said tube for impressing upon said anode a potential developed by a flow through said wall coating of current derived from said source of energy, and electronic means for varying the magnitude of the current flow, whereby to vary the potential impressed upon said anode.

2. In combination, a cathode ray tube having a source of electrons and an electron focusin anode, a source of electrical energy, a relatively high resistance wall coating for said tube having one end thereof electrically connected to said anode, electronic means connected to control a flow through said wall coating of current derived from said source of energy, and means for varying the conductivity of said electronic means to vary the magnitude of the current flow, whereby to vary the potential impressed upon said anode.

3. In a television receiver, a cathode ray tube having a source of electrons and an anode for focusing an electron beam, a source of electrical energy, a wall coating for said tube, said wall coating being of arelatively high resistance material and having one end electrically connected to said anode, a vacuum tube having its space discharge path connected to control a flow through said wall coating of current derived from said source of energy, and means for varying the conductivity of said vacuum tube to vary the magnitude of the current flow through said wall coating, whereby to vary the potential impressed upon said anode.

4. In a television receiver, a cathode ray'tube having a fluorescent screen and a source of electrons, an anode disposed intermediate of said fluorescent screen and said source of electrons for focusing an electron beam,'a source of electrical energy, a wall coating for said tube, said Wall coating being of a relatively high resistance material and having one end electrically connected to saidanode, a vacuum tube having its space discharge path connected in :series with said wall coating and said-source of energy, and means for varying the conductivity of said of a relatively high resistance material and havin one end electrically connected to said anode and the other end connected to one terminal of said source of energy, a vacuum tube having its space discharge path connected in series with said wall coating, and means for varying the conductivity of said vacuum tube to vary the magnitude of the current flow through said wall coating, whereby to vary the potential impressed upon said anode.

6. In a television receiver, a cathode ray tube having a fluorescent screen at one end and a source of electrons at the other end, an anode disposed intermediate of said fluorescent screen and said source of electrons for focusing an electron beam, 9, source of electrical energy, a wall adjacent said anode to a point adjacent said screen, said wall coating being of a relatively high resistance material and having one end electrically connected to said anode and the other end connected to one terminal of said source of energy, a Vacuum tube having its space coating for said tube extending from a point 6 discharge path connected between said anode and the other terminal of said source of energy, and means for varying the conductivity of said vacuum tube to vary the magnitude of the current flow through said wall coating, whereby to vary the potential impressed upon said anode MADISON CAWEIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 00., N. Y. city, pages 202, 203. 

